Active pixel sensor (APS) imaging devices are described in U.S. Pat. No. 5,471,515. These imaging devices include an array of pixel cells, arranged in rows and columns, that convert light energy into electric signals. Each pixel includes a photodetector and one or more active transistors. The transistors typically provide amplification, readout control and reset control, in addition to producing the electric signal output from the cell.
The pixels generate analog signals, which are converted into digital signals by analog-to-digital converters (ADCs) for further processing. The analog signal read-out chain may include a gain stage to amplify the analog signals into a range suitable for the ADCs. The magnitude of the gain may range from unity to about eight.
During readout, the amplifier provides adjustable signal gain with a given amplifier accuracy. The amplifier accuracy determines the required amplifier settling time, i.e., the interval between the application of the input voltage step and the point at which the output signal reaches and stays within a given error band.
In many sensors, the unity gain frequency of the amplifier is selected to satisfy the settling time requirement for the worst-case condition, i.e., the maximum signal gain. While this design approach ensures that the amplifier is complying with the settling time requirement at any signal gain selection, it only optimizes the amplifier power consumption in the unique case of the worst-case (maximum) gain setting. For any gain setting lower than the maximum gain setting, the amplifier draws more power than is necessary for the required settling time.